The present invention relates to a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, fitted with an alumina feed device for feeding alumina over substantially the entire surface of the molten electrolyte; an alumina feed device for such a cell; and a method for producing aluminium in such a cell.
The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 950xc2x0 C. is more than one hundred years old. This process, conceived almost simultaneously by Hall and Hxc3xa9roult, has not evolved as much as other electrochemical processes, despite the tremendous growth in the total production of aluminium that in fifty years has increased almost one hundred fold. The process and the cell design have not undergone any great change or improvement and carbonaceous materials are still used as electrodes and cell linings.
An important aspect of the production of aluminium in such cells resides in the way in which alumina is fed to the molten electrolyte for its subsequent dissolution and electrolysis, as described hereafter.
Conventional cells are usually operated with a crust of frozen electrolyte above the molten electrolyte. This crust needs to be periodically broken to form an opening for feeding alumina into the molten electrolyte situated underneath. Various systems have been provided to locally break the frozen electrolyte crust and feed alumina into the molten electrolyte, for instance as described in U.S. Pat. Nos. 3,664,946 (Schaper/Springer/Kyburz), 4,049,529 (Golla), 4,437,964 (Gerphagnon/Wolter), 5,045,168 (Dalen/Kvalavag/Nagell), 5,108,557 (Nordquist), 5,294,318 (Grant/Kristoff), 5,324,408 and 5,423,968 (both in the name of Kissane).
One drawback of feeding alumina to the molten electrolyte by initially breaking the electrolyte crust resides in the introduction of a mass of frozen electrolyte into the molten electrolyte which generates a thermal shock therein. Moreover, after the crust is broken cold alumina is added to the molten electrolyte which inevitably freezes the bath, thereby forming dense alumina and/or electrolyte aggregates increasing the chance of sludging.
Therefore, with the trend towards more automated systems, the frequency of feeding alumina has been increased. Feeding may take place every 20 to 90 min., sometimes even shorter, for instance every 1 to 5 min. as described in U.S. Pat. No. 3,673,075 (Kibby), while smaller amounts of alumina are introduced with each feed. The advantages are in particular maintaining a more constant concentration of dissolved alumina in the electrolyte and reducing the temperature variation in the electrolyte. A typical automated break and feed system comprises a pneumatically-operated crust breaker beam and an ore bin capable of discharging a fixed amount of alumina (K. Grjotheim and B. J. Welsh, xe2x80x9cAluminium: Smelter Technology xe2x80x9d, 1988, 2nd Edition, Aluminium Verlag GmbH, D-4000 Dxc3xcsseldorf 1, pp. 231-232).
U.S. Pat. No. 5,476,574 (Welsh/Stretch/Purdie) discloses a feeder arranged to continuously feed alumina to an aluminium electrowinning cell. The feeder is associated with a point breaker which is operated to maintain a hole in a frozen electrolyte crust on the surface of the molten electrolyte.
In order to enhance dispersion, dissolution and control of the amount of fine particulate alumina fed to the electrolytic bath, various alumina feed devices have been developed involving fluidisation of alumina powder by using compressed gas such as compressed air, for instance as disclosed in U.S. Pat. Nos. 3,901,787 (Niizeki/Watanabe/Yamamoto/Takeuchi/Kubota), 4,498,818 (Gudmundur/Eggertsson) and 4,525,105 (Jaggi).
Although substantial efforts have been made to enhance the feeding of alumina as described above, such feeding is still locally limited to one or more feeding points over the electrolytic bath between dipping carbon anode blocks. Furthermore, the above described processes still necessitate to periodically form or continuously maintain as many holes into the frozen electrolyte crust above the molten bath as there are feeding points.
It is an object of the invention to provide a cell for the electrowinning of aluminium fitted with an alumina feed device designed to feed alumina to substantially the entire anode""s surface.
A further object of the invention to provide a cell for the electrowinning of aluminium fitted with an alumina feed device designed to operate with a substantially crustless molten electrolyte.
Another object of the invention is to provide a cell for the electrowinning of aluminium fitted with an alumina feed device designed to feed and disperse pre-heated alumina powder to the molten electrolyte to minimise the risk of sludging and enhance dissolution of the fed alumina.
Yet another object of the invention is to provide a cell for the electrowinning of aluminium fitted with an alumina feed device designed to feed continuously or intermittently alumina to the molten electrolyte.
A still further object of the invention is to provide an alumina feed device for such aluminium electrowinning cells as well as a method to produce aluminium in such cells.
The invention relates to an electrolytic cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. The cell comprises a plurality of anodes immersed in the molten electrolyte, each anode having an oxygen-evolving active surface of open structure facing and spaced by an inter-electrode gap from a cathode; a thermal insulating cover above the molten electrolyte surface; and an alumina feed device arranged above the molten electrolyte surface for supplying alumina to the molten electrolyte surface from where the alumina is dissolved as it enters the electrolyte to enrich the electrolyte in dissolved alumina. Alumina-containing electrolyte is electrolysed in the inter-electrode gaps to produce oxygen gas on the anodes an aluminium on the cathode.
The alumina feed device comprises means for spraying and/or blowing alumina between the molten electrolyte surface and the thermal insulating cover and over an entire expanse of the surface of the electrolyte, hereinafter called the xe2x80x9calumina feeding areaxe2x80x9d, so that upon dissolution of alumina sprayed and/or blown to the electrolyte, electrolyte enriched in dissolved alumina flows to the inter-electrode gaps where is electrolysed.
In other words, the anode feeding area is at least a portion of the surface of the electrolyte whose size is substantially greater than that with conventional point feeders. Thus, alumina powder fed with this feeder is spread over a substantially greater surface of molten electrolyte and can much easier dissolve. The size of the expanse may be at least a tenth or a fifth of the surface area of the anode structure, in particular from a quarter to a half of the full surface area. Typically, the expanse may have a size of at least 0.1 m2, such as 0.5 or 1 or 2 m2 to 6 or 10 m2 or more.
Conveniently, the spraying and/or blowing means are arranged to spray and/or blow alumina into an area which corresponds approximately to the perpendicular projection on the surface of the molten electrolyte of the active anode surface. For example, a spraying and/or blowing means may be arranged to spray and/or blow alumina over an expanse which covers entirely or at least partly the perpendicular projection onto the molten electrolyte surface of an active anode surface. The alumina feeding area may correspond to the feeding area of one anode or several anodes.
In one embodiment, the anode feeding area corresponds to a projection onto the surface of the electrolyte of the active anode surfaces, this projection possibly being smaller or greater than the corresponding area(s) of the active anode surfaces. This anode feeding area is usually, but not necessarily, situated above the active anode surfaces.
The alumina feeding area typically occupies an expanse of the molten electrolyte surface which can be about the same size as the surface area of the corresponding active anode surfaces. However, when anodes co-operate with special electrolyte circulation means, for instance as disclosed in co-pending application PCT/IB00/00027 (de Nora), the size of the feeding area may be smaller than the actual size of the active anode surfaces. In practice, powder alumina may even be supplied over substantially the entire surface of the molten electrolyte.
This is particularly advantageous in configurations where at least part of the alumina-rich electrolyte flows through the open anode structures to the inter-electrode gap. At least part of the alumina-rich electrolyte may flow around the open anode structures into the inter-electrode gap to be electrolysed and then alumina-depleted electrolyte can rise to the feeding area through the open anode structures.
Whether or not alumina flows around the anodes, alumina dissolution is improved with such an alumina feeding device. The improvement is not bound to a specific electrolyte circulation path. Either alumina-rich electrolyte flows from the feeding area down through the anode structure, or alumina-depleted electrolyte flows through the anode structure up to the feeding area, or both flow patterns are combined.
Although the concept of this invention may be adapted to any aluminium electrowinning cell, it is specially designed for cells operating with metal-based anodes at reduced temperatures, typically below 910xc2x0 C., such as in the range of 730xc2x0 to 870xc2x0 C. or 750xc2x0 to 850xc2x0 C., in particular cells as disclosed in co-pending applications PCT/IB00/00029 and PCT/IB00/00027 (both in the name of de Nora) operating with metal-based oxygen-evolving grid-like anodes provided with vertical through openings for the circulation of electrolyte and the escape of anodically produced oxygen.
Suitable materials for oxygen-evolving anodes include iron and nickel based alloys which may be heat-treated in an oxidising atmosphere as disclosed in WO00/06802, WO00/06803 (both in the name of Duruz/de Nora/Crottaz), WO00/06804 (Crottaz/Duruz), PCT/IB99/01976 (Duruz/de Nora) and PCT/IB99/01977 (de Nora/Duruz). Further oxygen-evolving anode materials are disclosed in WO99/36593, WO99/36594, WO00/06801, WO00/06805, PCT/IB00/00028 (all in the name of de Nora/Duruz), WO00/06800 (Duruz/de Nora), WO99/36591 and WO99/36592 (both in the name of de Nora).
The thermal insulating cover is normally arranged to inhibit formation of an electrolyte crust on the surface of the molten electrolyte during operation. However, the surface of the electrolyte does not need to be entirely crust free, but at least the feeding area should be free from any frozen electrolyte crust for optimal operation.
Also, to overcome a prior art prejudice as described above, it is preferred to supply preheated alumina to the molten electrolyte to minimise electrolyte freezing caused by contact with xe2x80x9ccoldxe2x80x9d solid alumina and by the endothermic alumina dissolution reaction in the molten electrolyte. Ideally the fed alumina supplies at least part of the energy needed for its dissolution. Heat may be provided to the alumina during the feeding process by contact with hot air, by using a heater or possibly with a burner providing a flame which may also be used to spray and/or blow alumina powder. The alumina may be preheated before feeding, for instance by heating an alumina reservoir in which it is stored and from which it is fed to the molten electrolyte by spraying and/or blowing according to the invention. More generally, the alumina may be heated before and/or during spraying and/or blowing.
The alumina feed device may be fitted with a blower or a fan for spraying or blowing alumina with gas, e.g. air.
Bayer-process alumina or other suitable grades of alumina, may be utilised. For instance, partly dehydrated alumina particles, modified alumina, and alumina particles of different shapes and sizes may be used.
To enhance dispersion of the alumina powder above the molten electrolyte surface, and to facilitate its dissolution into the molten electrolyte, the alumina powder is preferably composed of particles in the range of 20 to 200 micron, preferably from 30 to 50 micron.
In one embodiment of the invention, the alumina feed device comprises nozzles for spraying alumina. Usually, a plurality of nozzles are distributed along at least one generally horizontal alumina feeding pipe that is arranged to carry alumina from an alumina reservoir to the nozzles. The nozzles may be placed in a generally horizontal sideways arrangement along the feeding pipe so as to generate a horizontal dispersion of sprayed alumina, to spray alumina powder over substantially the entire molten electrolyte surface.
The alumina feed device may comprise a blower or a fan, for spraying alumina from the nozzles with compressed gas, preferably hot gas, in particular hot air. The alumina may be preheated by using a radiator as described above.
In another embodiment, the alumina feed device may comprise an alumina reservoir for feeding alumina onto a spreader from which during operation alumina is sprayed and/or blown, for instance, such spreader may be a rotary spreader which rotates so as to spray the alumina by centrifugal force. The rotary spreader may comprise a substantially horizontal planar spreading surface, for instance substantially circular, and arranged to rotate in its own plane. Such spreaders may co-operate with a fan and/or a blower to blow alumina from the spreader with gas or a flame.
The invention relates also to an alumina feed device for feeding alumina to the surface of a fluoride-containing molten electrolyte of a cell for the electrowinning of aluminium from alumina dissolved in the molten electrolyte, in particular a cell comprising a thermal insulation above the surface of the molten electrolyte.
According to the invention, the alumina feed device comprises means for spraying and/or blowing alumina powder over an entire expanse of the surface of the molten electrolyte, as described above. Usually, the spraying and/or blowing means is arranged to spray and/or blow alumina sidewards, for example nozzles arranged substantially horizontally or a substantially horizontal alumina spreader as described above.
As opposed to conventional point feeders which feed alumina only to one point of the electrolyte surface, the alumina feeding device according to the invention is arranged to feed alumina powder over an entire expanse of the molten electrolyte surface which enhances the dissolution of fed alumina.
Furthermore, there is no need to remove the spraying and/or blowing means from under the insulating cover or possibly the crust of molten electrolyte. Normally the means is permanently located under the cover or the crust which can remain sealed off while alumina is fed to the molten electrolyte to avoid thermic losses. Conversely, conventional feeders are located above the crust of molten electrolyte, the crust being periodically broken to permit alumina feeding from above the crust into the molten electrolyte.
Another aspect of the invention is a method of producing aluminium in a cell as described above. The method comprises spraying and/or blowing alumina from the alumina feed device over an entire expanse of the surface of the molten electrolyte from where the alumina dissolves as it enters the electrolyte to enrich the electrolyte in dissolved alumina, feeding the electrolyte enriched with alumina to the inter-electrode gaps and electrolysing the electrolyte enriched with alumina in the inter-electrode gaps to produce oxygen on the active anode surfaces and aluminium on a facing cathode.
A further aspect of the invention is an electrolytic cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. The cell comprises one or more anodes immersed in a molten electrolyte, each anode having an oxygen-evolving active surface of open structure facing and spaced by an inter-electrode gap from a cathode; a thermal insulation above the molten electrolyte surface; and means for supplying alumina powder to the molten electrolyte surface from where the alumina is dissolved as it enters the electrolyte to enrich the electrolyte in dissolved alumina. Alumina-containing electrolyte is electrolysed in the inter-electrode gaps to produce oxygen gas on the anodes and aluminium on the cathode.
The means for supplying alumina powder is located above the molten electrolyte surface and extends through the thermal insulation. For instance, the alumina supply means comprises an alumina distribution head or nozzle or the like that extends through the thermal insulation. The alumina supply means is arranged for supplying alumina powder over an area of the surface of the electrolyte so that upon dissolution of alumina supplied to the electrolyte, electrolyte enriched in dissolved alumina flows down to the inter-electrode gaps where it is electrolysed. At least part of the electrolyte enriched in dissolved alumina flows down through the open anode structures to the inter-electrode gaps and/or alumina depleted electrolyte flows up from the inter-electrode gaps through the open anode structures.
Thus the openings in the anode structure are used for the down and/or up flow of electrolyte from and/or to the alumina feeding area.
Usually, the thermal insulation above the molten electrolyte consists of a cover which is placed and spaced above the surface of the molten electrolyte, for instance as disclosed in co-pending patent application WO99/02763 (de Nora/Sekhar). Such cover thermally insulates the surface of the molten electrolyte and substantially prevents formation of an electrolyte crust on the molten electrolyte. The thermally insulated cavity thereby created between the molten electrolyte and the cover serves to house the alumina supply means, in particular the spraying and/or blowing means, of the alumina feed device.
Alternatively, if the cell is operated at a conventional temperature (i.e. around 950xc2x0 C.) the thermal insulation can also include an electrolyte crust, formed by electrolyte freezing, but which is sufficiently spaced from the molten electrolyte to permit the insertion of the alumina supply means, in particular the spraying and/or blowing means, between the molten electrolyte and the crust, the molten electrolyte level being maintained at a sufficiently low level below the crust. However, cells operated at reduced temperatures (i.e. typically between 730xc2x0 and 870xc2x0 C. or between 750xc2x0and 850xc2x0 C.) should have an insulating cover above the molten electrolyte, since at such temperatures, the molten electrolyte does not usually form a rigid crust but a gel-like layer.